The Horse, Equus caballus, is a large ungulate mammal, one of the seven modern species of the genus Equus. It has been important for transportation; to ride on, or for pulling a chariot, carriage, horse-drawn boat, stagecoach, tram, etc.; also as plough horse, etc. as well as food; see also domestication of the horse. Until the mid 20th century, the horse was used heavily in warfare.

The evolution of the horse from the very early (around 55 million years ago) Hyracotherium or eohippus to the wild equids listed below, is well understood in comparison to our understanding of the evolutionary succession of most animals. By natural selection, the toes of early horse ancestors reduced to the single central toe which forms the hoof of the modern equine. The opposite being animals with 'cloven' hooves (2 toes), like cows, pigs and sheep. Vestiges of other toes remain as the splint bones, the callous-like "chestnuts" on the inner sides of all four legs, and the "ergots" hidden in the hair of the underside of the fetlock joint.

Horses live in family groups in primarily grassland habitats. These normally consist of a mature stallion, his harem of mares, and the mares' offspring. Once young males reach breeding age and begin to attempt to breed mares or challenge the herd stallion, they are driven out of the herd and form "bachelor bands" with other young stallions. It's usually not until a stallion reaches 7 or 8 years old that he stands a real chance at acquiring mares, eventually becoming, if successful in the attempt, a "band stallion", i.e. having a harem of his own, having separated female equids from another stallion's band.

An alpha mare dictates the direction in which a family herd travels, while the stallion brings up the rear, "herding" his family. Recently, researchers have observed that there seems to be a form of democracy among horses. For instance, if the majority of the herd decides it's time to stop and eat, the whole herd will stop and eat.

The earliest evidence for the domestication of the horse comes from Central Asia and dates to about 3,000 BCE. There are competing theories about the time and place of domestication. However, wild species continued into historic times, including the Forest Horse, Equus caballus silvaticus (also called the Diluvial Horse); it is thought to have evolved into Equus caballus germanicus, and may have contributed to the development of the heavy horses of northern Europe, such as the Ardennais.